Method and device for feeding back exhaust gas from a gas turbine with a downstream waste heat boiler

ABSTRACT

A method for feeding back exhaust gas from a gas turbine with a downstream waste heat boiler, wherein this exhaust gas is metered into the air inflow stream of a gas turbine, with the result that the temperature and the composition of the exhaust gas can be controlled, and in this way highly concentrated carbon dioxide (CO 2 ) is obtained which can be injected into a storage facility, so that the balance of the carbon dioxide for the entire process can be kept low or is negligible. The temperature in the gas turbine can be lowered and the proportion of carbon dioxide in the exhaust gas can be considerably increased. After combustion has taken place and heat has been exchanged, gas washing is possible, carbon dioxide can be recovered and, the proportion of free oxygen in the exhaust gas can be lowered.

The invention relates to a process for the recycling of waste gas from a gas turbine with downstream waste heat boiler, the said waste gas being metered to the supply air stream of a gas turbine in such a way that the temperature and the composition of the waste gas can be controlled and in this way highly concentrated carbon dioxide (CO₂) is obtained which can be injected into a storage site so that the carbon dioxide balance for the entire process can be kept low or is negligible. By metered recycling of the waste gas it is possible to decrease the temperature in the gas turbine and to considerably increase the carbon dioxide content in the waste gas so that, after combustion and heat exchange, gas scrubbing will be possible and, on the one hand, the carbon dioxide can be recovered and, on the other hand, the content of free oxygen in the waste gas can be decreased. In another embodiment of the invention an oxygen-enriched gas is fed together with a fuel gas to a gas turbine for combustion and then diluted with waste gas so that the temperature can be kept low despite oxygen enrichment, and, after combustion and heat exchange, a highly concentrated carbon dioxide is obtained.

Many processes for the generation of energy use the combustion of combustible gases in a gas turbine which converts the direct combustion energy to mechanical energy. The hot waste gases are then cooled in a heat exchanger, with steam being generated and used, in turn, for driving a second turbine which also generates mechanical energy. The mechanical energy can, in turn, be used for various purposes; it is frequently used for driving auxiliary units or for generating electric energy. Such processes which are frequently used in gas and steam power plants and operate by the principle of combined heat and power generation are of high efficiency.

As a fuel gas for such processes all gases that are suitable for driving gas turbines can be used, which are ultimately gases which can be fed to the gas chamber of a turbine and do not produce any corrosive residues or combustion products during combustion. These are, for example, natural gas, refinery gases, biogases or synthesis gases. Refinery gases are particularly understood to be such gases that form during the processing of liquid fossil fuels, such as butane, hydrogenous gases or liquid gas, also referred to as LPG (‘Liquefied Petroleum Gas’). If, for example, synthesis gas is used, it can be produced in any way. A process for the production of synthesis gas is, for example, the coal gasification where a finely ground carbonaceous fuel is gasified with an oxygenous gas in an entrained flow gasification. The synthesis gas thus produced can be used for driving gas turbines by means of combustion. To ensure the usability of the fuel gas in a gas turbine, gas scrubbing is normally carried out prior to combustion so that the fuel gas does not produce any corrosive gases during combustion and a cost-efficient service life of the gas turbine can be achieved.

The temperature in the combustion of fuel gases in gas turbines is normally up to 2200° C. After combustion the hot waste gas is fed to a waste heat boiler so that the sensible heat of the waste gas can be used for the recovery of steam. During combustion carbon dioxide (CO₂) and water (H₂O) form so that—except for these gases—the gas will only contain nitrogen (N₂) if the fuel gas is subjected to gas scrubbing prior to combustion. If pure oxygen is used for combustion, the waste gas virtually only contains carbon dioxide and water.

Carbon dioxide is a greenhouse gas which contributes to global warming. For this reason, many countries aim at keeping the amount of carbon dioxide emitted into the earth's atmosphere at a low level. It is therefore technically feasible to design processes in such a way that they produce less or no carbon dioxide right from the beginning. As the use of pure hydrogen as fuel gas is normally not cost-efficient, efforts are made to provide processes which render a low or negligible carbon dioxide emission, which is normally achieved by gas scrubbing. This process serves to remove the carbon dioxide from the combustion gases by absorption of the carbon dioxide with the aid of an absorbing solvent. The carbon dioxide is then recovered during the regeneration of the absorbing solvent.

In order to avoid discharge of the carbon dioxide obtained from gas scrubbing to the atmosphere, the carbon dioxide can be compressed and injected into a storage site. In this way, this gas is permanently kept from entering the atmosphere. An example of a process for the re-injection of compressed carbon dioxide into a storage site is given in EP1258595A2.

Even though such a re-injection of carbon dioxide into a storage site keeps the emission of carbon dioxide into the atmosphere low or at a negligible level, it reduces the cost-efficiency of the process. The gas scrubbing for the removal of carbon dioxide, the compression of the carbon dioxide, possible transport of the compressed carbon dioxide and the re-injection into a storage site result in additional costs which have an impact on the cost-efficiency of the process. For this reason, efforts are made to keep the costs for the additional process steps required for the downstream processing of the carbon dioxide as low as possible.

A starting point for this is to keep the composition of the waste gas from a gas turbine such that gas scrubbing requires as little effort as possible. This means primarily to keep the carbon dioxide content in the waste gas as high as possible so that gas scrubbing has to render only little enrichment. In addition, the oxygen content of the waste gas to be treated should be as low as possible because oxygen impairs the operability of most absorbing solvents. Many absorbing solvents used for the removal of carbon dioxide by gas scrubbing contain amino groups which react with oxygen. For this reason, the composition of the waste gas from a gas turbine is important for the cost-efficiency of the entire process.

It is therefore of advantage if a process for operating a gas turbine with downstream heat recovery system produces a waste gas which has a high carbon dioxide content and a very low oxygen content (O₂) right from the beginning. In addition, the content of nitrogen as ballast gas should be as low as possible. Other gases should also be present in minor quantities only. However, this is normally the case anyway if gas scrubbing is carried out prior to combustion and combustion is carried out stoichiometrically.

It is therefore the objective to provide a process which provides a highest possible carbon dioxide content in percent by volume and a lowest possible oxygen content in percent by volume. In addition, the process is to allow keeping the nitrogen content in percent by volume at a low level.

The present invention achieves this objective by a process which exists in two embodiments which, to a certain extent, represent peripheral areas of a main process step, this main process step consisting in metering a part-stream of the cooled waste gas leaving the waste heat boiler to the combustion air of the gas turbine after heat exchange so that a higher content of carbon dioxide is obtained and, after combustion, heat exchange for the recovery of thermal energy and gas scrubbing are carried out, carbon dioxide (CO₂) being obtained. This method, to a certain extent, represents a peripheral area, the other peripheral area consisting in avoiding gas scrubbing by using pure oxygen as oxidising agent in the gas turbine. As a result, only carbon dioxide and water are produced during combustion so that pure carbon dioxide (CO₂) is obtained after condensation of the water.

The waste gas is metered to the combustion air of the gas turbine in such a way that as much waste gas as possible is recycled but combustion can nevertheless be performed easily. The latter is preferably controlled on the basis of measuring parameters, a measuring parameter consisting in the measurement of the combustion temperature in the gas turbine. Proper handling of this method will yield a waste gas which will contain only little oxygen. It is also possible to use an oxygen-enriched gas for the combustion in the gas turbine and to carry out gas scrubbing after heat recovery. In this case, the oxygen content in the waste gas is advantageously maintained at such a level that there is no notable impairment to gas scrubbing.

In so doing, carbon dioxide is preferably obtained in high concentration. It can be pure or technically pure but can actually be of any concentration.

Claim is particularly laid to a process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine by burning a fuel gas suitable for combustion with an oxygenous gas in a gas turbine so that mechanical energy is generated and the waste gas evaporates water in a waste heat boiler by indirect heat exchange so that hot steam is generated, and which is characterised in that a part-stream of the cooled waste gas is metered to the combustion air of the gas turbine after having left the waste heat boiler, the said waste gas being fed to the gas turbine for combustion, and another part-stream of the cooled waste gas is fed to a gas scrubber for the absorption of acid gases after having left the waste heat boiler, with carbon dioxide (CO₂) being recovered from the said gas scrubber.

Claim is also laid to a process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine by burning a fuel gas suitable for combustion with an oxygenous gas in a gas turbine with an oxygen-enriched gas so that mechanical energy is generated and the waste gas evaporates water in a waste heat boiler by indirect heat exchange so that hot steam is generated, characterised in that a part-stream of the cooled waste gas is metered to the combustion air of the gas turbine after having left the waste heat boiler, and the other part-stream is cooled so that water condenses and carbon dioxide (CO₂) is recovered.

Processes for the use of gas turbines including the recycling of waste gas part-streams are basically known from EP0453059B1 or JP4116232A. The latter, however, do not include the recovery of carbon dioxide and do not meter the recycled waste gas.

The oxygen-enriched gas is preferably taken from an air separation unit. However, the said gas can also be provided by a pressure swing adsorption unit. The oxygen-enriched gas can actually be produced in any way desired. The use of an oxygen-enriched gas as oxidising agent in the gas turbine causes an increase of the carbon dioxide content after combustion and a decrease of the nitrogen content in the waste gas. Gas scrubbing is thus simplified as the gas ballast of the nitrogen during gas scrubbing is low. It will nevertheless be required if the nitrogen content in the carbon dioxide of the waste gas is technically existent. In one embodiment of the invention where oxygen-enriched combustion air is used, a part-stream of the cooled waste gas is fed to a gas scrubber for the absorption of acid gases after having left the waste heat boiler, with carbon dioxide (CO₂) being recovered from the said gas scrubber. When using an oxygen-enriched gas as oxidising agent combustion must be done properly by metering cooled waste gas in order to keep the residual oxygen content in the combustion at a low level.

In one embodiment of the invention the oxygen-enriched gas is pure oxygen, the other part-stream obtained being cooled so that water condenses and carbon dioxide (CO₂) is recovered. As in the case of the other embodiments the carbon dioxide may then be compressed and injected into the storage site. If pure oxygen is used, there is no nitrogen content in the waste gas. In this case there is no need for gas scrubbing.

The fuel gas for the gas turbine can be of any type as long as it is suitable for combustion in a gas turbine. In this context, it is particularly important that during combustion the fuel gas does not deliver any corrosive constituents which may affect the turbine. In one embodiment of the invention the fuel gas is synthesis gas.

In another advantageous embodiment the synthesis gas is a synthesis gas which originates from a coal gasification reaction in which a finely ground carbonaceous fuel gas is gasified with an oxygenous gas in an entrained flow reaction. Coal gasification reactions for the production of synthesis gas are well known from the state of the art; an exemplary embodiment of a coal gasification reaction for the recovery of synthesis gas is given in EP0616022B1.

However, the fuel gas can also be natural gas. Prior to combustion in a gas turbine, it can be treated so that corrosive constituents and particularly sulphur compounds are removed. An example of natural gas treatment is given in EP920901B1. The treated natural gas is then used for firing the gas turbine.

In another embodiment of the invention the fuel gas is a refinery gas. The treatment of liquid fossil fuels frequently yields gases which can be used for the heating of gas turbines. Examples are LPG (“Liquefied Petroleum Gas”), propanes and butanes and hydrogen. In an exemplary embodiment the latter can be added to the combustion gas of a gas turbine if it is intended to use the process embodying the invention.

In another embodiment of the invention the fuel gas is biogas. This is a fuel gas produced from biological raw materials such as wood, manure, straw or grasses. These may, for example, be obtained by fermentation but also, for example, by gasification.

The carbon dioxide obtained can then be compressed and injected into a carbon dioxide storage site. Although this is the preferred embodiment within the framework of the invention, it is just as well conceivable to use the carbon dioxide for other purposes or to use a part-stream for re-injection into a storage site.

Metering of the cooled and recycled waste gas from a gas turbine with waste heat boiler is preferably done on the basis of measured values. This is typically the temperature of the waste gas from the gas turbine directly downstream of the gas turbine and prior to entering the waste heat boiler. In one embodiment of the invention the portion of the recycled gas stream from the waste heat boiler and the amount of the part-stream metered into the gas turbine are hence controlled by the values measured for the temperature of the waste gas from the gas turbine. This is a preferred embodiment but it is also feasible, for example, to measure the gas constituents in the waste gas and to meter the cooled and recycled waste gas on the basis of these measured values. Gas constituents suitable for measurement are, for example, carbon dioxide (CO₂) or oxygen (O₂). Controlling is carried out either manually or by computer. Claim is also laid to a contrivance for running the process embodying the invention, provided there is a corresponding interconnection of plant sections.

With the aid of the gas turbine, mechanical energy is generated which can be used for any purpose. It can, for example, be used for the generation of electric power. The thermal energy from the waste heat boiler can also be used for any purpose. The latter can preferably be used for the generation of steam and, via a turbine, for the generation of electric power. In the process embodying the invention actually as many turbines as desired can be used.

The invention has the advantage to provide treated carbon dioxide (CO₂) from a gas turbine for compression and re-injection into a storage site, the cost efficiency of the process being improved by recycling a part-stream of the waste gas from the gas turbine in gas flow direction downstream of the waste heat boiler to the gas turbine and metering it to the combustion air so that the carbon dioxide content in the waste gas is increased in such a way that either gas scrubbing for the removal of carbon dioxide from the waste gas can be carried out in a cost efficient way or, in an ideal configuration, completely omitted by using an oxygen-enriched oxidising agent.

The invention is explained by means of two drawings showing exemplary embodiments only and not being restricted to the latter.

FIG. 1 shows the process embodying the invention in which a first part-stream of the waste gas is recycled downstream of the waste heat boiler and metered to the gas turbine, and the second part-stream of the waste gas is fed downstream of the waste heat boiler to a gas scrubber for carbon dioxide.

FIG. 2 shows the process embodying the invention in which a first part-stream of the waste gas is recycled downstream of the waste heat boiler and metered to the gas turbine which is heated with pure oxygen as oxidising agent, and the second part-stream of the waste gas condenses downstream of the waste heat boiler and is used as pure carbon dioxide stream.

FIG. 1 shows a gas turbine (1) heated with a carbonaceous fuel gas (2) and combustion air (3), the combustion air (3) being added via a mixing valve (4) and mechanical energy being generated by the combustion in the gas turbine (1). The waste gas (5) from the gas turbine (1) is fed to a waste heat boiler (6) where the waste gas (5) dissipates its sensible heat via indirect heat exchange to water (6 a) supplied and as a result steam (6 b) is generated. A part-stream of the waste gas (5 a) is recycled and added to the combustion air (3) via the mixing valve (4). As a result, the carbon dioxide content in the waste gas (5) increases. In addition, the temperature of the combustion gas and the waste gas (5) is lowered, this being of non-impairing effect on the gas turbine (1). The other part-stream of the waste gas (5 b) is fed to a gas scrubber (7) containing an absorbing solvent used to remove the carbon dioxide (CO₂, 8) by scrubbing, a tail gas free of carbon dioxide (7 a) being obtained. This is recovered during the regeneration (9) of the solvent and can be compressed and injected into a storage site. The recycled amount (5 a) is metered by controlling the mixing valve (4) on the basis of the measurement of the temperature of the waste gas (5) with the aid of a sensor (10) and is controlled by a computer (10 a).

FIG. 2 also shows a gas turbine (1) heated with a carbonaceous fuel gas (2) and pure oxygen (11) from an air separation unit (11 a) which separates the exhaust air (3 a) into oxygen (11) and the residual air constituents (11 b), with pure oxygen (11) being added as oxidising agent via a mixing valve (4) and mechanical energy being generated by the combustion in the gas turbine (1). The waste gas (5) from the gas turbine (1) is fed to a waste heat boiler where the waste gas (5) dissipates its sensible heat via indirect heat exchange to water (6 a) supplied and as a result steam (6 b) is generated. A part-stream of the waste gas (5 a) is recycled and added to the oxygen (11) via the mixing valve (4). As pure oxygen (11) is used as oxidising agent, the waste gas (5) contains only water (H₂O) and carbon dioxide (CO₂). The second part-stream (5 b) of the cooled waste gas is further cooled for condensation (5 c) so that virtually pure carbon dioxide (8) is obtained after separation of the condensed water (5 d). In addition, the temperature of the combustion gas and the waste gas (5) is lowered by the recycled amount, this being of non-impairing effect on the gas turbine (1). The carbon dioxide (8) can be compressed and injected into a storage site. The recycled amount is metered by controlling the mixing valve (4) on the basis of the measurement of the temperature of the waste gas (5) with the aid of a sensor (10) and is controlled by a computer (10 a).

LIST OF REFERENCE NUMBERS AND DESIGNATIONS

-   1 Gas turbine -   2 Carbonaceous fuel gas -   3 Combustion air -   3 a Air for the air separation unit -   4 Mixing valve -   5 Waste gas -   5 a First part-stream of the waste gas -   5 b Second part-stream of the waste gas -   5 c Cooler or condenser -   5 d Condensed water -   6 Waste heat boiler or heat exchanger -   6 a Water -   6 b Steam -   7 Gas scrubber -   8 Carbon dioxide (CO₂) -   9 Regeneration unit -   10 Temperature sensor -   10 a Computer -   11 Oxygenous gaseous oxidising agent -   11 a Air separation unit -   11 b Residual air constituents 

1. A process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine, in which a fuel gas suitable for combustion with an oxygenous gas is burnt in a gas turbine so that mechanical energy is generated and the waste gas evaporates water in a waste heat boiler by indirect heat exchange so that hot steam is generated, wherein a part-stream of the cooled waste gas is metered to the combustion air of the gas turbine after having left the waste heat boiler, this stream being fed to the gas turbine for combustion, and another part-stream of the cooled waste gas is fed to a gas scrubber for the absorption of acid gases after having left the waste heat boiler, with carbon dioxide CO₂ being recovered from the said gas scrubber.
 2. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine, in which a fuel gas suitable for combustion with an oxygenous gas is burnt with an oxygen-enriched gas in a gas turbine so that mechanical energy is generated and the waste gas evaporates water in a waste heat boiler by indirect heat exchange so that hot steam is generated, wherein a part-stream of the cooled waste gas is metered to the combustion air of the gas turbine after having left the waste heat boiler, and the other part-stream is cooled in a cooler so that water condenses and carbon dioxide is recovered.
 3. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 2, wherein the other part-stream of the cooled waste gas is fed to a gas scrubber for the absorption of acid gases after having left the waste heat boiler, with carbon dioxide being recovered from the said gas scrubber.
 4. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 2, wherein the oxygen-enriched gas is pure oxygen and the other part-stream is cooled so that water condenses and carbon dioxide is recovered.
 5. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 1, wherein the fuel gas is synthesis gas.
 6. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 5, wherein the synthesis gas originates from a coal gasification reaction in which a finely ground carbonaceous fuel gas is gasified with an oxygenous gas in an entrained flow reaction.
 7. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 1, wherein the fuel gas is natural gas.
 8. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 1, wherein the fuel gas is a refinery gas.
 9. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 1, wherein the fuel gas is biogas.
 10. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 1, wherein the carbon dioxide obtained is compressed and injected into a carbon dioxide storage site.
 11. The process for the metered recycling of cooled waste gas from the waste heat boiler of a gas turbine according to claim 1, wherein the portion of the recycled gas stream from the waste heat boiler and the amount of the part-stream metered into the gas turbine are controlled by the values measured for the temperature of the waste gas from the gas turbine. 